Restore the binary compatibility for link_map l_addr.

Restore the binary compatibility for link_map l_addr.

Keep link_map l_addr binary layout compatible, rename l_addr to l_base
where rtld returns map base.  Provide relocbase in newly added l_addr.

This effectively reverts the patch to the initial version of D24918.

Reported by: antoine (portmgr)
Reviewed by:	jhb, markj
Tested by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D24946